Our investigations of the structure and function of the tryptophan synthase multienzyme complex are providing insights into mechanisms of catalysis, metabolite channeling, and allosteric interactions. Crystal structures reveal ligand-induced conformational changes that convert tryptophan synthase from an open form to a closed form. Investigations of the effects of temperature, pH, and guanidine hydrochloride on activity and on spectroscopic properties provide further evidence for allosteric interactions that trigger the open to closed conformational transition. Mutation of residues in the interaction site between the alpha and beta subunits alters the transmission of allosteric signals. Cystationine beta-synthase (CBS) is a pyridoxal phosphate dependent enzyme that is closely related to tryptophan synthase. Mutations in human CBS cause the genetic disease homocystinuria. To understand the effects of disease causing mutations on CBS structure and function, we have subcloned and overexpressed the closely related yeast CBS (56 kDa) and an N-terminal 39 kDa truncated fragment (tCBS) and have purified both enzymes to homogeneity. Spectroscopic studies demonstrate formation of a key enzyme substrate-intermediate and establish the overall catalytic mechanism. - enzymology, pyridoxal phosphate, tryptophan synthase, multienzyme complex, mutation, crystallography, channeling, allostery, cystathionine beta-synthase